\b = search for a word boundry (so that if you entered the word "all" it won't match "small".

\Q = disable search metacharacters which have special meanings inside regular expressions so that if you wanted the word ".(" it would work without blowing up your script by making it look for the matching ")". See the perlre documentation page.

\E = re-enables the use of search metacharacters.

\b = another search boundry, so that the word "old" won't match "goldden".

Finally, /i make the search case-insensitive.

Two other things: there was a typo in the code which I'll fix after this ( /b should have been /\b ), and UBB did its usuall trick of inserting a space inbetween the &#0124; &#0124;'s so it may not have run beforehand.